Hole transport has been investigated in films of solid solutions of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine (TPD) in bisphenol A polycarbonate and polystyrene. Rather than being ‘inert’ as previously assumed, the binder plays a major role in influencing the rate of charge exchange between molecules. It is found that the absolute values of the drift mobilities, their electric-field dependence and the activation energies are strong functions of the binder polymer employed to cast the films. At equivalent low molecular concentrations, the hole mobility in films of molecular dispersions in polystyrene is two orders of magnitude higher than that observed in dispersions in polycarbonate, on which most investigations reported in the literature have been carried out. The increased mobility of dispersions in polystyrene is directly related to the reduction in the activation energies.